1. Field of the Invention
The present invention relates to an image forming method and an image forming apparatus. Specifically, the present invention relates to a method for developing an electrostatic latent image with a developer, and to a developing apparatus including magnetic poles for forming a magnetic brush of a developer in a developing region on a surface of a developer bearing member. In addition, the present invention also relates to a process cartridge which produces images using a toner, and a method for fixing a toner image.
2. Discussion of the Background
Copiers, printers, facsimile apparatus and similar electrophotographic or electrostatic image forming apparatus generally include a latent image bearing member such as photoconductive drums or photoconductive belts. A latent image is formed on the image bearing member in accordance with image data. It is popular to use a magnet brush developing method using a two-component developer made of a toner and a carrier from the view point of image transferability, halftone reproducibility and temperature/humidity stability of developing characteristics. In such a developing method, the two-component developer forms brush chains on a developer bearing member and is fed to a developing region where the developer bearing member faces the image bearing member. At the developing region, the toner in the developer adheres to an electrostatic latent image portion formed on the latent image bearing member.
The above-mentioned developer bearing member usually includes a cylindrical sleeve and a magnet roller located in the sleeve for forming a magnetic field by which a developer forms a magnet brush on the surface of the sleeve.
By rotating at least one of the above-mentioned sleeve and magnet roller, erected chains of the developer are moved on the surface of the sleeve. The developer conveyed to the developing region is erected along lines of the magnetic force caused by a main development magnetic pole. The brush chains contact the surface of the latent image bearing member while yielding, and the brush chains rub the latent image because of moving at a linear velocity different from that of the latent image bearing member. At this time, the developer provides the toner for the latent image, resulting in development of the latent image.
Published unexamined Japanese Patent Applications Nos. 2000-305360 and 2000-347506 have proposed image forming technologies to improve image quality of both a high density image portion and a low density image portion at the same time. It is disclosed therein a developing apparatus which visualizes an electrostatic latent image on an image bearing member and which includes a developing sleeve including a nonmagnetic sleeve, and a magnet roller fixedly set within the nonmagnetic sleeve and including plural magnets arranged at a regular angle, wherein the developing sleeve magnetically bears a magnetic two-component developer including a toner and a carrier to form a magnet brush thereon, and wherein the developing sleeve rubs the image bearing member with the magnet brush to visualize the electrostatic latent image at a rubbing region. In this developing apparatus, the attenuation ratio of a magnetic flux density at the rubbing region in a normal line direction is specified. In addition, the attenuation ratio of magnetic flux densities of a main magnet and a magnet adjacent thereto at the rubbing region in a normal line direction, or an angle between the main magnet and the magnet adjacent to the main magnet at the rubbing region are specified.
However, in such a high-efficiency developing method in which a magnetic force of a main development magnetic pole is high, and a developer having a short length of magnet brush rubs a surface of a photoreceptor at a rotating speed of from 1.1 to 3.0 times that of the photoreceptor, the toner is insufficiently supplied i.e., the resultant images have a low image density or the resultant images are unclear when the rotating speed is less than 1.5 times. Therefore the rotating speed ratio is preferably not less than 1.5 times. In this case, a rear-end omission problem in that the rear end of a solid image is omitted occurs. Such a problem tends to be caused under a condition in which the rotating speed ratio is greater than 1.0. This problem is seriously caused as the rotating speed of the magnetic brush increases.
In order to prevent the rear-end omission problem of toner in such a developing process, i.e., in order to obtain satisfactory image density and image qualities, it is necessary to improve developing ability by another method.
Currently, a toner having a smaller particle size is desired to produce high quality images.
When the particle size of a toner is miniaturized, the content of fine particles in the toner increases. It is confirmed by experiment that a toner having a small particle diameter remarkably contaminates a developing sleeve. The mechanism of this phenomenon is as follows. Toner particles present on a portion of a developing sleeve corresponding to a non-image portion of a photoreceptor is pushed toward the developing sleeve by an electric field. Normally, the toner particles are quickly re-adhered to a surface of the carrier due to electrostatic attraction. However, since fine toner particles have extremely bad fluidity (characteristic specific to a fine powder), the fine toner particles adhered to the, developing sleeve are hardly re-adhered to the carrier surface. Namely, adhesion strength of find toner particles against the developing sleeve is extremely strong. Furthermore, when the fine toner particles adhered to the developing sleeve are rubbed with the carrier many times, fusion-bonding of the toner to the sleeve occurs (hereinafter this phenomenon is referred to as on-sleeve toner fixation). When this on-sleeve toner fixation occurs, the image density decreases with time. In particular, when a solid image is printed continuously on four sheets of paper after a 100,000-copy running test, it is found that the image density of the solid image gradually decreases from the first sheet to the fourth sheet. Namely, since an electrically insulating layer constituted of a toner ingredient is formed on a surface of an electroconductive sleeve, the effective bias of the developing bias applied lowers, and thereby the developing ability of the developing sleeve is deteriorated.
Published unexamined Japanese Patent Application No. 2000-10336 proposes that a developing sleeve is subjected to a blast treatment with spherical particles to form smooth unevenness portion thereon in order to prevent adhesion of a toner to a developing sleeve. The adhesion of a toner to a sleeve can be prevented to some extent by this method, but the ability of the sleeve to feed a developer is not enough for current fast printing machines, and high quality images cannot be obtained easily.
On the other hand, a wax is conventionally included in a toner in order to impart a releasing property to the toner at fixation.
Since waxes have a smaller molecular weight and is softer than a binder resin, so-called a filming phenomenon in that the waxes adhere to a carrier and a photoreceptor and thereby a wax film is formed thereon tends to occur. When the filming phenomenon occurs on the carrier (i.e., a spent carrier problem), the toner cannot be friction-charged with such a carrier. As a result, defective charging occurs and the resultant images have background fouling. In addition, a white stripe abnormal image appears on a halftone image when a wax film is formed on a photoreceptor. In addition, waxes tend to cause the on-sleeve toner fixation. These phenomena turn worse, i.e., it is difficult to maintain the initial image qualities, when copying processes are repeated.
In addition, it is known that temperature increases in a developing apparatus with repetition of copying processes, resulting in increase of the atmospheric temperature at a nip region. The heat is easy to stay in the above-mentioned developer, which has a high density of brush chains, i.e., the heat tends to hardly leak from the developing region. As a result, the wax in the toner easily bleeds out, resulting in occurrence of fixation of the wax on the sleeve, and filming of the wax on the photoreceptor and the carrier used.
Because of these reasons, a need exists for an image forming method by which high quality images are stably produced for a long period of time.